Interest in telescopes continues to increase as both professional and amateur astronomers make new discoveries. Optical filter assemblies for telescopes have demanding requirements for stability as well as optical quality, due to the fact that long viewing times are often employed to observe quiescent solar features. This places additional demands on the safety, stability and physical construction of the filter assemblies.
Solar telescopes have the additional requirement that discrete, weak signals must be discriminated against an extremely bright background. First, this is needed to protect the eyes of the viewer, or to avoid saturation and overload of a detector. Second, many events of interest, such as solar flares or filaments, are transient, and would be lost against the bright background signal of the solar luminance without additional filtering. Third, filters that selectively block the high background signal are desired so the observer may discern more details in discrete or transient events.
Narrow bandpass filters and spectrophotometric techniques can be used to block some of the background signal, and aid discrimination of discrete events. Spectrophotometric techniques are particularly suitable for instrumental detection systems, and narrow bandpass filters are more practical for direct telescope viewing. A narrow bandpass filter is precisely designed to control (either block or transmit) light in an extremely narrow wavelength region. For instance, for solar event observation, a wavelength region of particular interest is the Hα atomic line emission centered at 656.28 nm. Hydrogen accounts for more than 91% of the composition of the sun, and this emission line is quite prevalent. Because this atomic emission line is in the visible spectrum, it is readily accessible to human observation—if the bright and extensive background spectrum ranging from the deep ultraviolet (UV) through the far infrared (IR) region can be blocked. Hence, combinations of filters are typically employed to remove exposure to excess light from outside the region of interest, and selectively pass light from the chosen band to the eye.
Narrow bandpass filters are often made by thin film technologies that combine multilayer stacks of coatings to create constructive or destructive interference with the light impinging on the filter. Spacer layers are employed to further narrow the bandpass, but deposited spacer layers can become increasingly lossy, and affect the overall performance of the filter. Therefore, another approach is to form an etalon, in which a physical spacer layer is interposed between reflectors or interference stacks. Etalons fall broadly into two categories: air-gapped, or solid-state.
Air-gapped etalons have been widely used for decades in research telescopes as well as other optical applications. Two plates coated with selective reflectors or other coatings are held at a controllable distance from each other. In principle, this distance can be adjusted to tune the etalon onto the precise narrow spectral band of interest—at least within a narrow tuning window. In practice, the difficulty is to maintain this desired distance, and therefore the spectral position of the narrow bandpass filter, in the face of barometric pressure changes, temperature changes, or even typical transport or alignment changes as one tracks across the sky. If the spacing changes even slightly, the interference design can be detuned to the extent that the narrow band of interest is now blocked from observation. In addition, the coated plates are typically extremely thin optical materials and therefore fragile. Hence, active control of temperature or pressure is typically combined with this technology, by placing the assembly in a heated case, or by more involved pressure control methods such as those described in U.S. Pat. Nos. 4,204,771 and 6,269,202, incorporated by reference. Other methods of tuning and stabilizing the etalon, such as those described in U.S. Pat. Nos. 5,710,655 and 6,452,725, which are incorporated by reference as if fully set forth herein, add specific, additional layers to the etalon that will alter the etalon's composite refractive index and therefore bandpass location in response to variable voltage or thermal changes. Such control systems add complexity, bulk and cost, and may have slow response times to changing conditions.
Solid-state etalons use an extremely high quality crystalline material or optical flat as the “spacer”. Thus, the solid-state etalon has a more robust foundation than a variable air gap design. The required etalon coatings are then formed on the outside surfaces of the solid spacer material. This means the spectral range of the narrow bandpass will be more stable to pressure and temperature changes, because it is physically held in position by the solid spacer. However, one can easily understand that significant expense, skill and time may be required to create a spacer of sufficient flatness, transmission and purity to support the reflector layers. In addition, if the manufactured bandpass is even fractionally off-alignment from the wavelength of interest, the etalon must be physically tilted to change the effective pathlength through the filter, and this may alter its aperture or aspect ratio in an optical assembly. Nonetheless, such tilt control can be implemented easily and reproducibly, and actually may be used advantageously to redirect stray reflections out of the optical path.
In any optical assembly, there will be concern with overall light throughput. Often solid-state etalons suffer from absorption losses due to impurities or irregularities in the crystal structure, or to their inherent optical properties. Hence, one commonly chooses materials with the highest transmittance, so that subsequent optical losses throughout the system can be better accommodated. For instance, while mica cleaves cleanly along parallel crystal plates, it is difficult to control reproducibly, and may contain inclusions. Moreover, mica has lower transmissivity than silica, lithium niobate, or other crystals, especially the less expensive and more common varieties of mica. This has lead to the continued development of carefully controlled synthesized crystals rather than the use of natural materials. Unfortunately, such synthesis is typically accompanied by higher costs. Nonetheless, with the increased emphasis on high transmissivity starting materials to counteract other system losses, this extra cost is commonly perceived as a necessity for the required optical performance, and the lower transmissivity mica materials typically have been avoided as unsuitable for high precision use.
While the etalon defines the narrow bandpass of interest, for practical applications, one combines the etalon with other filters to further enhance rejection of stray, excess and extraneous light. This is particularly important in sensitive detection systems because etalons tend to transmit light from multiple reflections within the spacer of the etalon, giving rise to closely spaced “orders” of passbands. Hence, combinations of etalons with various, broader band blocking filters or rejection filters have been reported since at least the 1960s, and used in telescopes, including the Hα telescope designed for the Skylab mission, remote sensing systems, and spectral analysis.
Exemplary systems often include rejection filter(s) to block light well away from the spectral region of interest (UV and/or IR filters, for instance) and furthermore, use additional visible light filters to remove or block higher order overtone transmissions.
For instance, U.S. Pat. No. 4,092,070, which is incorporated by reference as if fully set forth herein, uses a tunable acousto-optic filter in combination with an etalon to actively select bandpasses. U.S. Pat. No. 5,125,743, which is incorporated by reference as if fully set forth herein, describes a sophisticated system for measuring solar magnetic fields by actively tuning an etalon between different bandpasses. In this complex system, a first bandpass is chosen to infer the solar magnetic field from a given atomic emission, while a second bandpass is selected to provide data on the direction and intensity of the field in order to combine information and construct the solar magnetic vector maps. The system further employs multiple additional filters, including a narrow bandpass filter to selectively block other wavelengths away from the atomic emission, and requires rotatable or variable polarization analysers to characterize multiple polarization states of transmitted light for additional magnetic field information. This illustrates another innovative approach to the use of etalons, and their versatility in combination with other optical elements to create highly sensitive and selective detection systems.
In addition, optical systems using etalons to define narrow bandpasses of light may incorporate additional filters and techniques, such as those described in U.S. Pat. Nos. 5,287,214 and 5,781,268, which are incorporated by reference, and which are specifically designed to recombine polarization states separated by the etalon. These techniques are designed to make the systems insensitive to polarization effects and increase total light throughput by additively combining different polarization states. Such techniques as described require filters or techniques that can provide at least two changes in polarization state, so that any one state may be transformed into its complementary state and continue along the optical path.
While solar telescopes require sufficient light throughput for observation, they have the additional requirement of maximum rejection of stray light caused by reflections off the filters, and excess, extraneous light. Too much filtering may isolate the exact bandpass desired, but not with enough intensity for viewing. Too little filtering and the signal is lost in the high background noise of the solar spectrum. Furthermore, the viewer or detector can be irreparably harmed by excess radiation. Thus, such telescopes have unique and demanding requirements for careful filtering.
Recent telecommunications advances have lead to further improvements in narrow bandpass technologies. Multiplexed communication channels have similar requirements for high stray light rejection plus optimal and highly selective throughput of extremely narrow bandpasses. These techniques and capabilities may enhance performance in non-related fields such as solar telescopes.
Accordingly, improved optical filter assemblies for solar telescopes that couple advances in filter design and capabilities with cost-effective construction are desired.